Gray marks on contacts

[lucaslenar]

I was cleaning the corrosion out on the motherboard of a phone that went for a swim :/ When I unpluged the cameras there was corrosion which came off with cleaning. There was however these gray-brown marks left on the copper contacts. Are these burn marks or what.
They don't come off with an eraser.
The phone is a Samsung galaxy j3

 

[Aeacus]

Looks like completely normal aging process for copper since it does change color in due time,
further reading: https://www.crescentcitycopper.com/why-does-copper-turn-green.htm